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Datastore Loading Error: Slices of Slices in Nested Structs

When attempting to load datastore entities from a Python project into a Go project, you may encounter the error: "datastore: flattening nested structs leads to a slice of slices: field "Messages"". This occurs when the loaded entities contain a data model in Python that has nested structures and repeated fields, while Go doesn't allow multiple levels of slices within structs.

Data Model Definitions

In the Python model, the ModelA class defines a list of messages as a LocalStructuredProperty with repeated ModelB, while in Go, the ModelA struct has a slice of ModelB as the Messages field.

Python:

<code class="python">class ModelA(ndb.Model):
    ...
    messages = ndb.LocalStructuredProperty(ModelB, name='bm', repeated=True)</code>

Go:

<code class="go">type ModelA struct {
    ...
    Messages []ModelB `datastore:"bm,"`
}</code>

Troubleshooting

The error arises because Go doesn't allow for nested slices in structs. Hence, you need to modify your data structure to conform to Go's requirements.

Options:

• Don't Use Go: Avoid this error by performing the data loading in Python.
• Custom Deserializer: Implement your own datastore deserializer to handle the case of nested slices. However, this approach is not recommended due to its complexity.
• Data Structure Modification: Restructure your data in Python to avoid nested slices and slices of slices. Then, rewrite the data in your Go project to load the new structures.

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